Replace constant calls to black box functions with their result

[TomAFrench]

Problem

Consider the Noir program

use dep::std;

fn main() -> pub [u8; 32]  {
    std::hash::keccak256([1], 1)
}

This currently results in the ACIR

Compiled ACIR for main:
current witness index : 34
public parameters indices : []
return value indices : [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34]
EXPR [ (-1, _1) 1 ]
EXPR [ (-1, _2) 1 ]
BLACKBOX::KECCAK256 [(_1, num_bits: 8), (_2, num_bits: 32)] [ (_3,...,_34)]

We're then including a keccak256 constraint which needs to be proven with a whopping 21553 gates. This is unnecessary as we can bake in this knowledge at compile time, this circuit should instead just assign the return witnesses to the expected values without performing the hash.

Happy Case

We should add an logic in simplify_call so that if we have a black box function with all constant inputs then we simplify it to the black box function's outputs.

noir/crates/noirc_evaluator/src/ssa_refactor/ir/instruction.rs

Line 390 in 720b1be

fn simplify_call(func: ValueId, arguments: &[ValueId], dfg: &mut DataFlowGraph) -> SimplifyResult {

Alternatives Considered

No response

Additional Context

Some black box functions rely on the backend to provide a value so we cannot perform this optimisation for them currently.

Would you like to submit a PR for this Issue?

Yes

Support Needs

No response

[kevaundray]

See here for a small discussion on this: https://github.com/noir-lang/noir/pull/1896/files#r1258872431